Due to the high cost of oil and the inherent dangers associated with nuclear energy, engineers and scientists are again turning to coal as an inexpensive and readily available source of energy. Coal is burned to generate heat which is captured as steam in packaged boilers. The steam is either used to power a turbine which turns a generator and produces electricity or used in industrial process applications. Unfortunately, the burning of coal produces SO.sub.x and NO.sub.x which is extremely hazardous to the environment.
Conventional coal boilers are equipped with pulverized coal dispersion burners. Referring to FIG. 3 attached hereto, burners 1, 2, and 3 receive coal from coal bunker 4 and classifier 5. The coal is first pulverized into fine particles by a crusher or mill 6 which is powered by motor 18. The fine coal particles are delivered to the burners (1,2,3) by means of conduit 7 which includes coal fan 8 and coal valve 9. Combustion air is delivered to the burners (1,2,3) via forced draft fan or blower 10. The fine coal particles are propelled from the burners (1,2,3) into combustion chamber 11, wherein the coal is burned and the heat generated therefrom is used to produce steam. Substantial heat transfer occurs at platen 12 and superheater 13. Thereafter, the flue gases pass through economizer 14, wet scrubber 15 and stack 16. The flue gases which exit wet scrubber 15 are directed to stack 16 via induced draft fan or blower 17.
The burners used in conventional pulverized coal boilers are inefficient insofar as they use a lot of power, require expensive coal pulverization, necessitate the application of expensive wet scrubbers to reduce SO.sub.x emissions, and produce flue gases with undesirably high NO.sub.x levels. Furthermore, NO.sub.x levels are typically reduced via selective catalytic reduction by injecting ammonium or urea, but ammonia may occur and the additive solutions are expensive.
In an effort to overcome the above-mentioned disadvantages of conventional pulverized coal burners and to provide a more environmentally acceptable means for producing energy from coal, the present inventor has investigated the use of rotating fluid bed combustors as burners in coal fired boiler applications.
Early research on rotating fluid bed combustors is set forth in U.S. Pat. No. 4,039,272 (Elliot), which issued Aug. 2, 1977, and articles by C. I. Metcalfe and J. R. Howard, "Fluidisation and Gas Combustion in a Rotating Fluidised Bed", Applied Energy, Applied Science Publishers Ltd., Vol. 3, (1977), pp. 65-73, and J. Broughton and D. E. Elliott, "Heat Transfer and combustion in Centrifugal Fluidized Beds", I.Chem. E. Symposium Series No. 43, pp. 11-1 to 11-6.
All of the aforementioned rotary fluidized bed combustors were directed to combustion of coal in drums which rotated about their vertical axis so as to form,,substantially vertical beds. U.S. Pat. No. 4,039,272 (Elliott) discloses an apparatus for carrying out a reaction in a fluidized bed comprising a rotatable drum with a circumferential wall which is permeable to gases. A bed of particles is supported on the circumferential wall of the rotating drum during operation and the reactants are fed into the bed. A fluidizing gas is passed through the circumferential wall of the drum. A receiver is provided to retain small particles carried from the bed by the fluidizing gas. The small particles are returned to the bed when operation ceases. A reservoir for discharging larger particles into the bed after operation has commenced may be provided.
When the drum is rotated, the particles of which the bed is composed tend to move away from the axis of rotation towards the circumferential Wall. If the fluidizing gases are admitted to the bed through the circumferential Wall, the centrifugal action opposes the action of the gases on the particles and, provided the drum is rotated at a sufficiently high speed, the particles will not be blown completely away from the circumferential wall be fluidizing gases entering the bed at high speed. Variable speed drive means may be provided for rotating the drum. This arrangement enables the apparatus to operate satisfactorily with widely different rates of supply of combustible material and oxidant.
The apparatus according to the Elliott patent may also be used for gasification of coal, in which case air would be fed into the drum at a rate lower than that necessary to provide for complete combustion of the fuel. Gases leaving the drum through the outlet would then contain combustible gases.
The Metcalfe et al. article proposed that fuel flow rates in conventional fluidized bed combustors are limited by the maximum fluidization velocity at which the bed can operate without particles being elutriated from the bed; whereas a rotating fluidized bed combustor would allow the operating range to be extended by imposing a high centripetal acceleration on the particles which allows the bed to operate with higher fluidization velocities.
The Broughton article proposed that combustion intensity can be accomplished by means of rotating a cylindrical bed about its axis thus providing a centrifugal field which allows the fluidizing velocity for a given particle size to be increased more or less in proportion to the number of gravities imposed.
The aforementioned references disclose only vertically disposed rotary fluid bed combustors. The present inventor has developed a novel rotary fluid bed gasifier for use as novel burners attached to a boiler or furnace. This burner is extremely efficient and produces far less environmentally hazardous emissions then conventional burners.
Additional advantages of the present invention shall become apparent as described below.